Arachnoid cyst

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A Yorkshire Terrier with an arachnoid cyst, showing dilation of the quadrigeminal cistern and compressive effect of the quadrigeminal cyst[1]

Arachnoid cysts (intra-arachnoid cysts, meningeal cysts, leptomeningeal cysts, and arachnoid diverticula) are a rare congenital neurological disease of dogs[2][3][4].

These congenital defects resemble type III spinal meningeal cysts in humans. A number of breeds appear predisposed, including the Rottweiler[5], Schipperke[6], Yorkshire Terrier and Shih Tzu[7], suggesting an inherited etiology.

The cysts are characterized as cerebrospinal fluid (CSF)-filled, dorsal midline, intradural, extramedullary cavitational lesions associated with coarse arachnoid trabeculation. They result in spinal cord compression in the cervical vertebrae and brain[8]. The cystic cavities are reportedly separated from the compressed spinal cord by an intact pia mater. In one series of affected dogs, numerous blood vessels with enlarged perivascular spaces were noted caudal to the cyst[9]. A cranial opening to the cysts, presumably continuous with the subarachnoid space and allowing flow of CSF, has been noted.

Usually there is no evidence of inflammation within the meninges or tissues lining the cysts, however, a mixed cellular inflammatory reaction and reactive connective tissue proliferation was reported in the cyst wall surgically removed in one case. In this dog, the cyst wall was composed of piaarachnoid meningothelial cells. Many animals are less than 1 year of age, but cysts have been reported in animals up to 12 years of age. Onset of signs between 11 and 24 months was reported in one series of cases. Arachnoid cysts have been observed mainly in rostral cervical and caudal thoracic/thoracolumbar sites; however, multiple cysts in the caudal cervical area of three Rottweiler dogs have been observed. Cysts may also be multilobed or bilobed[10].

The cause of this condition is unknown[11] although congenital spinal dysraphism due to failure of fusion of the neural crest has been suggested, and indeed, in people, primary arachnoid cysts are regarded as a developmental abnormality of the arachnoid. Ultrastructurally, the cyst is formed by splitting of the arachnoid membrane and the wall of the cyst is independent of the inner layer of the dura mater. The wall of the cyst consists of an outer collagenous membrane and an inner layer of cells that appear similar to normal arachnoid cells.

In dogs, arachnoid cysts reportedly may occur secondary to trauma, infection, inflammation, or subarachnoid hemorrhage[12]. While no evidence of trauma, other diseases, or malformations was found in Frykman’s series of cases, disk herniation was thought to contribute to cyst formation in at least one dog in the study by Rylander’s group[13].

Clinical syndromes will reflect the location of the lesion. To date, cervical syndromes and thoracolumbar syndromes have been observed. Pain is usually not a feature. Curiously, behavioral changes of depression and aggression in one Rottweiler with a C2 - C3 arachnoid cyst, disappeared following surgery, suggesting possible relief of pain. Scoliosis has been reported in one dog[14]. Analysis of CSF is usually normal, although changes may include mild protein and/or mononuclear cell increase. Survey radiographs tend to be non-diagnostic, although vertebral canal enlargement, possibly secondary to pressure atrophy of bone caused by the cyst, may be found in some cases. Additionally, presence of spinal curvature, such as scoliosis, can be detected.

Diagnosis can be made using myelography, since the cysts usually fill with contrast agent, and there is often partial blockage to flow of the contrast medium, with associated moderate to severe (usually dorsal) spinal cord compression. The cysts often appear as drop-shaped or oval contrast-filled cavities. Computed tomography provides additional information on localization and lateralization of the cyst, and allows measurement of the degree of spinal cord compression. Cysts may extend over several spinal cord segments. Magnetic resonance imaging may identify spinal cord parenchymal changes, such as presence of syringomyelia. Sonography can be used to define the cyst wall, to characterize the internal architecture of the cyst wall, and to orientate the surgeon to the location and extent of the cyst[15].

Surgical decompression of the spinal cord appears to be the treatment of choice, using dorsal laminectomy or hemilaminectomy, in association with durotomy, drainage and/or partial excision of the cyst (surgical fenestration), and dural marsupialization often leads to permanent clinical improvement in a majority of dogs[16][17]; however, recurrences can occur and neurologic deficits may persist. Long-term follow-up studies (e.g., up to 4 years post-surgery) suggest that durectomy around the border of the cyst and dissecting it free from pia mater may give a more permanent recovery than durotomy and drainage. Medical treatment alone, using a decreasing anti-inflammatory dosage of prednisolone, was reported to be successful in one dog. Arachnoid cysts have also been reported in brainstem locations, including the pineal region and quadreminal system of dogs[18].

References

  1. Reed S et al (2009) Quadrigeminal arachnoid cysts in a kitten and a dog. J Vet Diagn Invest 21(5):707-710
  2. Vite, Ch (2004) Developmental disorders. In: Braund's Clinical Neurology in Small Animals: Localization, Diagnosis and Treatment. IVIS, Ithaca, New York, USA
  3. Gage ED, Hoerlein BF, Bartels JE. (1968) Spinal cord compression resulting from a leptomeningeal cyst in the dog. J Am Vet Med Assoc 152:1664-1670
  4. Parker AJ, Adams WM, Zachary JF. (1983) Spinal arachnoid cysts in the dog. J Am Anim Hosp Assoc 19:1001-1008
  5. Rylander H, Lipsitz D, Berry WL, et al (2002) Retropective analysis of spinal arachnoid cysts in 14 dogs. J Vet Intern Med 16:690-696
  6. Frykman OF. (1999) Spinal arachnoid cyst in four dogs: diagnosis, surgical treatment and follow-up results. J Small Anim Pract 40:544-549
  7. Ness MG. (1998) Spinal arachnoid cysts in two shih tzu littermates. Vet Rec 142:515-516
  8. LeCouter RA, Grandy JL. Diseases of the spinal cord. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. 5th ed. Philadelphia: WB Saunders Co, pp:608-657
  9. Dyce J, Herrtage ME, Houlton JEF, et al (1991) Canine spinal "arachnoid cysts". J Small Anim Pract 32:433-437
  10. Rylander H, Lipsitz D, Berry WL, et al (2002) Retropective analysis of spinal arachnoid cysts in 14 dogs. J Vet Intern Med 16:690-696
  11. Summers B, Cummings J, de Lahunta A. (1995) Veterinary Neuropathology. St Louis: Mosby, pp:351-401
  12. Harding B, Copp A. (1997) Malformations. In: Graham DI, Lantos P, eds. Greenfield's neuropathology. 6th ed. London: Arnold, pp:397-533
  13. Rylander H, Lipsitz D, Berry WL, et al (2002) Retropective analysis of spinal arachnoid cysts in 14 dogs. J Vet Intern Med 16:690-696
  14. Bagley RS, Silver GM, Seguin B, et al (1997) Scoliosis and associated cystic spinal cord lesion in a dog. J Am Vet Med Assoc 211:573-575
  15. Galloway AM, Curtis NC, Sommerlad SF, et al (1999) Correlative imaging findings in seven dogs and one cat with spinal arachnoid cysts. Vet Radiol Ultrasound 40:445-452
  16. Dyce J, Herrtage ME, Houlton JEF, et al (1991) Canine spinal "arachnoid cysts". J Small Anim Pract 32:433-437
  17. McKee WM, Renwick PW. (1994) Marsupialisation of an arachnoid cyst in a dog. J Small Anim Pract 35:108-111
  18. Vernau KM, Kortz GD, Koblik PD, et al (1997) Magnetic resonance imaging and computed tomography characteristics of intracranial intra-arachnoid cysts in 6 dogs. Vet Radiol Ultrasound 38:171-176
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